This invention relates to non-destructive sensing of pavement density. More specifically, the invention relates to use of capacitive proximity sensing to determine pavement material density.
In paving, asphalt pavement is often used. In the asphalt paving process, various grades of aggregate are used. The aggregate is mixed with asphalt cement (tar). There are also air voids within the mix. A hot asphalt mix as laid has more air voids than is desirable. A paver lays down the asphalt mix and levels the asphalt mix with a series of augers and scrapers. The material as laid is not dense enough due to the number of air voids in the asphalt mix. Therefore, a roller makes a number of passes over the layer of asphalt material (mat), driving back and forth, or otherwise creating sufficient compaction to form asphalt of the strength needed for the road surface.
There are a number of problems associated with this process of paving. These problems relate to determining when there is sufficient compaction of the asphalt. If the asphalt is not sufficiently compacted, the quality of the pavement suffers. The amount of compaction is dependent upon a number of factors including the asphalt cement content, the aggregate type, the aggregate size, the aggregate shape, the aggregate texture, the distribution gradation, filler content, the mix temperature, the speed of the roller, the number of passes of the roller, irregularities in the roller performance, and other factors.
It is important for the asphalt mat to be of proper density. If there is not proper density, there will be long term deterioration when the asphalt begins to crack or there may be other problems related to stability and durability.
The density of the asphalt mat is increased through repeated compression with the roller. Preferably, the asphalt is uniformly dense throughout. There are numerous problems associated with determining the density of the asphalt so that one can be assured of the resulting stability of the asphalt mat.
One method of determining density is through use of a nuclear gage. In this process, a number of different measurements are taken from different locations on the asphalt mat. Nuclear density measurement systems suffer from any of the following; high cost, safety issues of radioactive materials, need to drive a probe or spike into the mat; thereby leaving a hole in the mat, inability to measure density on a continuous basis, unfeasibility of installing a plurality of such machines on a piece of road construction equipment.
Other approaches have also been used to measure density. One such approach is a Capacitance Energy Dissipation (CED) method. The CED approach involves using a capacitor charged to a voltage to energize a plate in contact with the asphalt. Direct current then flows from the capacitor into the ground as the capacitor discharges. The time required to discharge the capacitor is measured and this time is compared to the discharge rate of a reference capacitor with a known discharge rate. There are also a number of problems associated with this method. One problem is that this method is dependent upon temperature. Another problem is that the capacitor plate must come in contact with the pavement, so that the contact plate would need to be periodically cleaned. Another problem is that there is a time associated with the discharge rate that may impede the ability to continuously measure pavement density.
Therefore there are a number of problems associated with measuring pavement density. Thus, it is a primary object of the present invention to provide an improved system and method for measuring pavement density.
Another object of the present invention is to provide a system and method for measuring pavement density that does not require a sensor to come into contact with the pavement.
Another object of the present invention is to provide a system and method for measuring pavement density that provides for non-destructive evaluation.
Yet another object of the present invention is to provide a system and method for measuring pavement density that is more sensitive to density of the top portion of the mat.
A further object of the present invention is to provide a system and method for measuring pavement density that reduces cost and complexity.
Another object of the present invention is to provide a system and method for measuring relative pavement density.
These and other objects of the present invention will become apparent from the following description.
The invention is a system and method for capacitive sensing of pavement material density. One or more capacitive proximity sensors are positioned adjacent to but not in contact with a pavement material such as an asphalt mat. The measurements from the sensors are correlated to known particular asphalt densities. Based on these test density readings, it is known whether further compaction is required.